1. Field of the Invention
This invention relates to thermal cracking of Diels-Alder adducts to form a thermally cracked (cracked) product that contains at least the original compounds from which the adducts were formed, and recovering at least one of the thus formed original compounds from the cracked product.
More particularly this invention relates to the foregoing cracking process wherein the heat for the cracking is provided by controlled cavitation.
2. Description of the Prior Art
This invention is applicable to the thermal cracking of Diels-Alder reaction products in general. However, for sake of clarity and brevity this invention will largely be described with respect to cyclopentadiene (CPD), C5H6.
The Diels-Alder reaction is a useful reaction that is widely employed using conjugated dienes (the original diene compound) and an unsaturated compound (the original dieneophile compound) having a carbonyl group and/or a methylene group. It is an addition reaction in which the addition of an ethylenic group in the original dieneophile compound adds across the 1, 4 position of the original diene compound.
The Diels-Alder reaction is well known. It is a second order reaction that is carried out by simply bringing the two original compounds together in the presence or absence of a solvent at temperatures ranging from room temperature to about 390° F. The reaction is normally exothermic and yields in many situations are essentially quantitative.
CPD is a well-known chemical building block as is its dimer, dicyclopentadiene (DCPD), C10H12. Both are used extensively in industry.
DCPD exists in two stereoisomeric forms, endo and exo isomers, but the product predominantly used in commerce is the endo isomer. This invention is applicable to either isomer. DCPD (4, 7-methano-3a, 4, 7, 7a-tetrahydroindene) is the form in which CPD is normally commercially marketed.
CPD is a very robust chemical with wide applicability in industry. This is due to its conjugated double bonds coupled with an active methylene group. Thus, CPD can undergo a diene addition reaction with almost any unsaturated compound. CPD even forms adducts with itself and its oligomers as is demonstrated hereinbelow.
However, attention should first be drawn to the dimer DCPD. CPD oligomerizes spontaneously to DCPD at ordinary (room or ambient) temperature to DCPD without need of a catalyst or other aid. That is why CPD is normally sold in the form of DCPD. Above 212° F. CPD substantial and significant noncatalytic polymerization of CPD to its tri, tetra, and higher polymers can be achieved. The formation of DCPD from CPD, and the subsequent formation of higher oligomers and polymers thereof involves just a series of Diels-Alder reactions. For example, the dimer is formed by adding 1 mole of monomer to a second mole of monomer, while the trimer is formed by adding the monomer to the dimer, and so on.
It is convenient to employ CPD in the dimer form because the monomer and dimer are easy to separate by simple distillation, CPD having a boiling point of 106.7° F. and DCPD having a boiling point of 338° F. Cracking of DCPD can be achieved by boiling DCPD at ambient pressure. As DCPD boils at about 338° F., it cracks at a rate of about 36% per hour. By maintaining a distillation overhead temperature at from about 105° F. to about 108° F. essentially pure CPD can be obtained.
With its two conjugated double bonds CPD readily undergoes diene addition across its 1, 4 carbon atom position with a dieneophile. As a result of this reaction and the robustness of CPD as an original reactant compound, innumerable Diels-Alder adducts can be made from this single conjugated diene compound. For example, using just CPD as the conjugated diene in the reaction, the dieneophile that can be reacted with this single compound can be at least one of dibasic acids and derivatives such as chloromaleic anhydride, maleic anhydride, etc.; monobasic acids such as crotonic acid, methacrylic acid, etc.; aldehydes such as acrolein, crotonaldehyde, etc.; ketones such as propenyl methyl ketone, vinyl methyl ketone, etc.; ketene; vinyl compounds such as ethylene, styrene, vinyl acetate, allene, etc.; acetylenes such as acetylene, acetylenedicarbonitrile, etc.; quinones such as p-benzoquinone; nitroso compounds such as nitrosobenzene; and on and on. Hence the description of this invention in its broadest sense is cracking Diels-Alder adducts, with the detailed description of this invention being directed for sake of brevity largely to CPD and DCPD.
CPD is produced from a variety of thermal operations such as coal carbonization (tar, light oil, coke-oven gas) and thermal cracking of hydrocarbons (gas oil, naphtha, propane, ethane, and the like). CPD is recovered conventionally from other hydrocarbons by distilling such other hydrocarbons in a manner such that a distillate comprising C5 hydrocarbons and lighter is formed. The distillate is heated at a temperature of about 212° F. to convert CPD to DCPD in a heat soaking operation that takes from about 5 to about 24 hours for reasons that will be explained hereinafter. The DCPD, which boils at a higher temperature than the unreacted hydrocarbons of the distillate, is recovered as distillation bottoms.
The dimer, DCPD, is the normal form in which CPD is made commercially available since CPD spontaneously reacts without help under ordinary conditions of temperature and pressure to form the dimer. CPD is formed by back cracking DCPD to CPD under elevated temperature, see U.S. Pat. No. 2,831,904 to Kreps.
Heretofore back cracking of DCPD to CPD to recover CPD has been carried out by heating the DCPD or DCPD containing stream at a temperature sufficient to back crack the DCPD, but, since DCPD is so reactive towards forming higher polymers, at the same time using a temperature and pressure sufficiently low to allow CPD to boil off while minimizing the formation of deposits of unwanted polymer(s) (polymer fouling). In addition to cracking DCPD by boiling same, vapor phase DCPD cracking can be employed. For example, DCPD can be vaporized and this vapor subjected to an elevated temperature of 600 to 700 degrees F. to thermally crack the DCPD to CPD. This high temperature vapor cracking process allows for a shorter cracking residence time.
Even when the back cracking temperature is kept low and a very long soaking time is tolerated, polymer fouling still occurs because when the reactive DCPD first hits a hot metal surface of the heat exchanger and furnace tubes employed in the back cracking process, gums (polymers) are formed on these hot surfaces and build up throughout even low temperature, long term cracking processes. Thus, unwanted polymers deposit on DCPD back cracking equipment is an omnipresent problem that hurts process efficiency and is costly to remove after the process is completed. Also, such unwanted polymers do not necessarily deposit in their entirety on hot equipment surfaces. Instead, some of these polymers can become dispersed in by-product streams of the process in question. When this occurs, because of its polymer content, the by-product stream can be severely degraded in its commercial value.
By this invention DCPD and many other Diels-Alder adducts can be back cracked to their original compounds without the need for extended soaking periods and without frequent and costly equipment cleaning to remove the polymer fouling currently endured by the prior art.